Light-emitting element, light-emitting device, display device, electronic device, and lighting device

ABSTRACT

An object is to provide a light-emitting element which includes an exciplex being used as an energy donor capable of efficiently transferring energy to a substance exhibiting thermally activated delayed fluorescence. The exciplex comprises two kinds of substances and its singlet and triplet excited states are close to each other. Thus, by making light emission of the exciplex overlap with an absorption band on the longest wavelength side which corresponds to absorption by the substance exhibiting thermally activated delayed fluorescence, i.e., an energy acceptor, in a singlet excited state, it becomes possible to achieve efficient energy transfer from a singlet excited state of the exciplex to a singlet excited state of the substance exhibiting thermally activated delayed fluorescence, and it also becomes possible to achieve efficient energy transfer from a triplet excited state of the exciplex to a triplet excited state of the substance exhibiting thermally activated delayed fluorescence.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a light-emitting element, a displaydevice, a light-emitting device, an electronic device, and a lightingdevice each of which includes an organic compound as a light-emittingsubstance.

2. Description of the Related Art

In recent years, research and development have been extensivelyconducted on light-emitting elements using electroluminescence (EL). Ina basic structure of such a light-emitting element, a layer containing alight-emitting substance (an EL layer) is interposed between a pair ofelectrodes. By applying voltage to this element, light emission from thelight-emitting substance can be obtained.

Since such a light-emitting element is of self-light-emitting type, thelight-emitting element has advantages over a liquid crystal display inthat visibility of pixels is high, backlight is not required, and so onand is therefore suitable as flat panel display elements. In addition,it is also a great advantage that a display including such alight-emitting element can be manufactured as a thin and lightweightdisplay. Furthermore, very high speed response is also one of thefeatures of such an element.

Since a light-emitting layer of such a light-emitting element can beformed in the form of a film, planar light emission can be achieved.Therefore, large-area light sources can be easily formed. This featureis difficult to obtain with point light sources typified by incandescentlamps and LEDs or linear light sources typified by fluorescent lamps.Thus, light-emitting elements also have great potential as planar lightsources which can be applied to lighting devices and the like.

In the case of an organic EL element in which an EL layer containing anorganic compound as the light-emitting substance is provided between apair of electrodes, application of a voltage between the pair ofelectrodes causes injection of electrons from the cathode and holes fromthe anode into the EL layer having a light-emitting property, and thus acurrent flows. By recombination of the injected electrons and holes, theorganic compound having a light-emitting property is put in an excitedstate to provide light emission.

The excited state of an organic compound can be a singlet excited stateor a triplet excited state, and light emission from the singlet excitedstate (S*) is referred to as fluorescence, and light emission from thetriplet excited state (T*) is referred to as phosphorescence. Thestatistical generation ratio of the excited states in the light-emittingelement is considered to be S*:T*=1:3. Therefore, a light-emittingelement including a phosphorescent compound capable of converting thetriplet excited state into light emission has been actively developed inrecent years.

However, most phosphorescent compounds currently available are complexescontaining a rare metal such as iridium as a central metal, which raisesconcern about the cost and the stability of supply. Therefore, asmaterials which do not contain a rare metal and can convert a tripletexcited state into light emission, materials exhibiting delayedfluorescence have been studied.

Patent Documents 1 and 2 disclose a material exhibiting thermallyactivated delayed fluorescence (TADF) (hereinafter also referred to as aTADF material) with relatively high efficiency even at low temperature.

REFERENCE

[Patent Document 1] Japanese Published Patent Application No.2004-241374

[Patent Document 2] Japanese Published Patent Application No.2006-024830

SUMMARY OF THE INVENTION

For a layer responsible for light emission in a light-emitting element,a so-called host-guest structure in which a light-emitting substance isdispersed as guest molecules in host molecules is commonly employed, forthe purpose of preventing concentration quenching or controlling thelight-emitting position or for reasons such as poor film quality of alight-emitting substance or the like. In that case, electron-holerecombination occurs partly in the light-emitting substance but mainlyin a host material. In order to improve emission efficiency, energytransfer from host molecules to guest molecules should be taken intoconsideration.

However, the TADF material disclosed in Patent Documents 1 and 2, inwhich TADF occurs with high efficiency, differs from ordinarylight-emitting substances and host molecules in the positionalrelationship between single and triplet excited states; the singlet andtriplet excited states of the TADF material are close to each other.Therefore, efficient energy transfer is difficult to achieve usingordinary host molecules.

In view of the above, an object of one embodiment of the presentinvention is to provide a light-emitting element which includes a TADFmaterial as a light-emitting substance and has high emission efficiency.Another object of one embodiment of the present invention is to providea light-emitting device, a display device, an electronic device, and alighting device each having reduced power consumption by using the abovelight-emitting element.

It is only necessary that at least one of the above objects be achievedin the present invention.

The present invention provides a light-emitting element in which anexciplex (an excited complex) is used as an energy donor capable ofefficiently transferring energy to a substance exhibiting thermallyactivated delayed fluorescence. The exciplex is formed from two kinds ofsubstances and is characterized in that its singlet and triplet excitedstates are close to each other. Thus, by making light emission of theexciplex overlap with an absorption band on the longest wavelength sidewhich corresponds to absorption by the substance exhibiting thermallyactivated delayed fluorescence, i.e., an energy acceptor, in a singletexcited state, it becomes possible to achieve efficient energy transferfrom a singlet excited state of the exciplex to a singlet excited stateof the substance exhibiting thermally activated delayed fluorescence,and it also becomes possible to achieve efficient energy transfer from atriplet excited state of the exciplex to a triplet excited state of thesubstance exhibiting thermally activated delayed fluorescence.

That is, one embodiment of the present invention is a light-emittingelement which includes a pair of electrodes and an EL layer between thepair of electrodes. The EL layer includes at least a light-emittinglayer. The light-emitting layer contains at least a first organiccompound, a second organic compound, and a light-emitting substance. Acombination of the first organic compound and the second organiccompound forms an exciplex. The light-emitting substance exhibitsthermally activated delayed fluorescence.

Another embodiment of the present invention is a light-emitting elementwhich includes a pair of electrodes and an EL layer between the pair ofelectrodes. The EL layer includes at least a light-emitting layer. Thelight-emitting layer contains at least a first organic compound, asecond organic compound, and a light-emitting substance. A combinationof the first organic compound and the second organic compound forms anexciplex. The light-emitting substance exhibits thermally activateddelayed fluorescence. An absorption band, on the lowest energy side, ofthe substance exhibiting thermally activated delayed fluorescenceoverlaps with a light emission spectrum of the exciplex.

Another embodiment of the present invention is a light-emitting elementhaving the above structure, in which the difference in equivalent energyvalue between a peak wavelength in the absorption band, on the lowestenergy side, of the substance exhibiting thermally activated delayedfluorescence and a peak wavelength of the light emission of the exciplexis 0.2 eV or less.

Another embodiment of the present invention is a light-emitting elementhaving the above structure, in which the difference in equivalent energyvalue between peak wavelengths of fluorescent light emission andphosphorescent light emission of the substance exhibiting thermallyactivated delayed fluorescence is 0.2 eV or less.

Another embodiment of the present invention is a light-emitting elementhaving the above structure, in which the difference in equivalent energyvalue between peak wavelengths of fluorescent light emission andphosphorescent light emission of the exciplex is 0.2 eV or less.

Another embodiment of the present invention is a light-emitting elementhaving the above structure, in which the substance exhibiting thermallyactivated delayed fluorescence is a heterocyclic compound having aπ-electron rich heteroaromatic ring and a π-electron deficientheteroaromatic ring.

Another embodiment of the present invention is a light-emitting elementhaving the above structure, in which the substance exhibiting thermallyactivated delayed fluorescence is a heterocyclic compound having aπ-electron rich heteroaromatic ring and a π-electron deficientheteroaromatic ring which are directly bonded to each other.

Another embodiment of the present invention is a light-emitting device,a display device, an electronic device, and a lighting device eachincluding a light-emitting element having the above structure.

Note that the light-emitting device in this specification includes, inits category, an image display device with a light-emitting element.Further, the category of the light-emitting device in this specificationincludes a module in which a light-emitting element is provided with aconnector such as an anisotropic conductive film or a TCP (tape carrierpackage); a module in which the end of the TCP is provided with aprinted wiring board; and a module in which an IC (integrated circuit)is directly mounted on a light-emitting element by a COG (chip on glass)method. Furthermore, the category includes light-emitting devices thatare used in lighting equipment or the like.

One embodiment of the present invention provides a light-emittingelement having high emission efficiency. By using the light-emittingelement, another embodiment of the present invention provides alight-emitting device, a display device, an electronic device, and alighting device each having reduced power consumption.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A and 1B are schematic diagrams of a light-emitting element.

FIG. 2 illustrates energy transfer in a light-emitting layer.

FIGS. 3A and 3B are schematic diagrams of an active matrixlight-emitting device.

FIGS. 4A and 4B are schematic diagrams of a passive matrixlight-emitting device.

FIGS. 5A and 5B are schematic diagrams of an active matrixlight-emitting device.

FIG. 6 is a schematic diagram of an active matrix light-emitting device.

FIGS. 7A and 7B are schematic diagrams of a lighting device.

FIGS. 8A, 8B1, 8B2, 8C, and 8D each illustrate an electronic device.

FIG. 9 illustrates an electronic device.

FIG. 10 illustrates a lighting device.

FIG. 11 illustrates a lighting device.

FIG. 12 illustrates in-vehicle display devices and lighting devices.

FIGS. 13A to 13C illustrate an electronic device.

FIGS. 14A and 14B are diagrams illustrating emission wavelengths ofexciplexes.

FIG. 15 shows current density-luminance characteristics of alight-emitting element 1 and a comparative light-emitting element 1.

FIG. 16 shows voltage-luminance characteristics of the light-emittingelement 1 and the comparative light-emitting element 1.

FIG. 17 shows luminance-current efficiency characteristics of thelight-emitting element 1 and the comparative light-emitting element 1.

FIG. 18 shows luminance-power efficiency characteristics of thelight-emitting element 1 and the comparative light-emitting element 1.

FIG. 19 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 1 and the comparative light-emitting element1.

FIG. 20 shows emission spectra of the light-emitting element 1 and thecomparative light-emitting element 1.

FIG. 21 shows current density-luminance characteristics of alight-emitting element 2 and a comparative light-emitting element 2.

FIG. 22 shows voltage-luminance characteristics of the light-emittingelement 2 and the comparative light-emitting element 2.

FIG. 23 shows luminance-current efficiency characteristics of thelight-emitting element 2 and the comparative light-emitting element 2.

FIG. 24 shows luminance-power efficiency characteristics of thelight-emitting element 2 and the comparative light-emitting element 2.

FIG. 25 shows luminance-external quantum efficiency characteristics ofthe light-emitting element 2 and the comparative light-emitting element2.

FIG. 26 shows emission spectra of the light-emitting element 2 and thecomparative light-emitting element 2.

DETAILED DESCRIPTION OF THE INVENTION

Embodiments of the present invention will be described below withreference to the drawings. Note that the present invention is notlimited to the following description, and it will be easily understoodby those skilled in the art that various changes and modifications canbe made without departing from the spirit and scope of the presentinvention. Therefore, the present invention should not be construed asbeing limited to the description in the following embodiments.

Embodiment 1

In a light-emitting element in which a substance exhibiting thermallyactivated delayed fluorescence (TADF) is used as a light-emittingsubstance, light emission occurs through the following energeticprocess. Note that a molecule providing excitation energy (an energydonor) is referred to as a host molecule, while the substance exhibitingTADF (an energy acceptor) is referred to as a guest molecule.

(1) The case where an electron and a hole are recombined in a guestmolecule, and the guest molecule is excited (direct recombinationprocess).

(1-1) When the excited state of the guest molecule is a singlet excitedstate, the guest molecule emits fluorescence.

(1-2) When the excited state of the guest molecule is a triplet excitedstate, the guest molecule undergoes reverse intersystem crossing to asinglet excited state by absorbing energy (mainly heat) and emitsfluorescence.

In the direct recombination process in (1), when the efficiency ofreverse intersystem crossing and the fluorescence quantum yield of theguest molecule are high, high emission efficiency can be obtained. Notethat it is preferable that the level of singlet excitation energy (S₁level) and the level of triplet excitation energy (T₁ level) of the hostmolecule be higher than the S₁ level and the T₁ level of the guestmolecule. Regarding the substance exhibiting TADF and having highreverse intersystem crossing efficiency (the guest molecule), the abovepatent documents and a variety of reports have been published.

(2) The case where an electron and a hole are recombined in a hostmolecule and the host molecule is put in an excited state (energytransfer process).

(2-1) When the excited state of the host molecule is a singlet excitedstate and the S₁ level of the host molecule is higher than the S₁ levelof the guest molecule, excitation energy is transferred from the hostmolecule to the guest molecule, and thus, the guest molecule is put in asinglet excited state. The guest molecule in the singlet excited stateemits fluorescence. Note that since direct transition of the guestmolecule from a singlet ground state to a triplet excited state isforbidden, energy transfer from the S₁ level of the host molecule to theT₁ level of the guest molecule is unlikely to be a main energy transferprocess; therefore, a description thereof is omitted here. In otherwords, energy transfer from the host molecule in the singlet excitedstate (¹H*) to the guest molecule in the singlet excited state (¹G*) isimportant as represented by Formula (2-1) below (where ¹G represents thesinglet ground state of the guest molecule and ¹H represents the singletground state of the host molecule).¹ H* ¹ G→ ¹ H+ ¹ G*  (2-1)

(2-2) When the excited state of the host molecule is a triplet excitedstate and the T₁ level of the host molecule is higher than the S₁ leveland T₁ level of the guest molecule, excitation energy is transferredfrom the T₁ level of the host molecule to the T₁ level of the guestmolecule, and thus, the guest molecule is put in a triplet excitedstate. The guest molecule in the triplet excited state undergoes reverseintersystem crossing by absorbing thermal energy and emits fluorescence.

In other words, as in Formula (2-2) below, energy is transferred fromthe host molecule in the triplet excited state (³H*) to the guestmolecule in the triplet excited state (³G*), and then the singletexcited state (¹G*) of the guest molecule is generated by reverseintersystem crossing.³ H*+ ¹ G→ ¹ H+ ³ G*→(Thermal energy)→(Reverse intersystem crossing)→¹H+ ¹ G*  (2-2)

When all the energy transfer processes described above in (2) occurefficiently, both the triplet excitation energy and the singletexcitation energy of the host molecule are efficiently converted intothe singlet excited state (¹G*) of the guest molecule. Thus,high-efficiency light emission is possible. In contrast, before theexcitation energy of the host molecule is transferred to the guestmolecule, when the host molecule itself is deactivated by emitting theexcitation energy as light or heat, the emission efficiency isdecreased.

Next, factors controlling the above-described processes ofintermolecular energy transfer between the host molecule and the guestmolecule are described. As mechanisms of the intermolecular energytransfer, two mechanisms, i.e., Förster mechanism and Dexter mechanism,have been proposed.

In Förster mechanism (dipole-dipole interaction), energy transfer doesnot require direct contact between molecules and energy is transferredthrough a resonant phenomenon of dipolar oscillation between a hostmolecule and a guest molecule. By the resonant phenomenon of dipolaroscillation, the host molecule provides energy to the guest molecule,and thus, the host molecule is put in a ground state and the guestmolecule is put in an excited state. Note that the rate constantk_(h*→g) of Förster mechanism is expressed by Formula (1).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 1} \right\rbrack & \; \\{k_{h^{*}->g} = {\frac{9000c^{4}K^{2}{\phi ln}\; 10}{128\pi^{5}n^{4}N\;{\tau R}^{6}}{\int{\frac{{f_{h}^{\prime}(v)}ɛ_{g}(v)}{v^{4}}{\mathbb{d}v}}}}} & (1)\end{matrix}$

In Formula (1), ν denotes a frequency, f′_(h)(ν) denotes a normalizedemission spectrum of a host molecule (a fluorescent spectrum in energytransfer from a singlet excited state, and a phosphorescent spectrum inenergy transfer from a triplet excited state), ε_(g)(ν) denotes a molarabsorption coefficient of a guest molecule, N denotes Avogadro's number,n denotes a refractive index of a medium, R denotes an intermoleculardistance between the host molecule and the guest molecule, τ denotes ameasured lifetime of an excited state (fluorescence lifetime orphosphorescence lifetime), c denotes the speed of light, φ denotes aluminescence quantum yield (a fluorescence quantum yield in energytransfer from a singlet excited state, and a phosphorescence quantumyield in energy transfer from a triplet excited state), and K² denotes acoefficient (0 to 4) of orientation of a transition dipole momentbetween the host molecule and the guest molecule. Note that K²=⅔ inrandom orientation.

In Dexter mechanism (electron exchange interaction), a host molecule anda guest molecule are close to a contact effective range where theirorbitals overlap, and the host molecule in an excited state and theguest molecule in a ground state exchange their electrons, which leadsto energy transfer. Note that the rate constant k_(h*→g) of Dextermechanism is expressed by Formula (2).

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 2} \right\rbrack & \; \\{k_{h^{*}->g} = {\left( \frac{2\pi}{h} \right)K^{2}{\exp\left( {- \frac{2R}{L}} \right)}{\int{{f_{h}^{\prime}(v)}{ɛ_{g}^{\prime}(v)}{\mathbb{d}v}}}}} & (2)\end{matrix}$

In Formula (2), h denotes a Planck constant, K′ denotes a constanthaving an energy dimension, ν denotes a frequency, f′_(h)(ν) denotes anormalized emission spectrum of a host molecule (a fluorescent spectrumin energy transfer from a singlet excited state, and a phosphorescentspectrum in energy transfer from a triplet excited state), ε′_(g)(ν)denotes a normalized absorption spectrum of a guest molecule, L denotesan effective molecular radius, and R denotes an intermolecular distancebetween the host molecule and the guest molecule.

Here, the efficiency of energy transfer from the host molecule to theguest molecule (energy transfer efficiency Φ_(ET)) is thought to beexpressed by Formula (3). In the formula, k_(r) denotes a rate constantof a light-emission process (fluorescence in energy transfer from asinglet excited state, and phosphorescence in energy transfer from atriplet excited state) of a host molecule, k_(n) denotes a rate constantof a non-light-emission process (thermal deactivation or intersystemcrossing) of a host molecule, and τ denotes a measured lifetime of anexcited state of a host molecule.

$\begin{matrix}\left\lbrack {{Formula}\mspace{14mu} 3} \right\rbrack & \; \\{\Phi_{ET} = {\frac{k_{h^{*}->g}}{k_{r} + k_{n} + k_{h^{*}->g}} = \frac{k_{h^{*}->g}}{\left( \frac{1}{\tau} \right) + k_{h^{*}->g}}}} & (3)\end{matrix}$

According to Formula (3), it is found that the energy transferefficiency Φ_(ET) can be increased by increasing the rate constantk_(h*→g) of energy transfer so that another competing rate constantk_(r)+k_(n)(=1/τ) becomes relatively small.

(Energy Transfer Efficiency in (2-1))

The energy transfer process in (2-1) is considered. In the process inFormula (2-1), energy transfers by both Förster mechanism (Formula (1))and Dexter mechanism (Formula (2)) are possible.

First, an energy transfer by Förster mechanism is considered. When τ iseliminated from Formula (1) and Formula (3), it can be said that theenergy transfer efficiency Φ_(ET) is higher when the quantum yield φ(here, a fluorescent quantum yield because energy transfer from asinglet excited state is discussed) is higher. However, in practice, amore important factor is that the emission spectrum of the host molecule(here, a fluorescent spectrum because energy transfer from a singletexcited state is discussed) largely overlaps with the absorptionspectrum of the guest molecule (absorption corresponding to thetransition from the singlet ground state to the singlet excited state)(note that it is preferable that the molar absorption coefficient of theguest molecule be also high). This means that the fluorescent spectrumof the host material overlaps with the absorption band of the substanceexhibiting TADF, i.e., the guest material, which is on the longestwavelength side.

Next, an energy transfer by Dexter mechanism is considered. According toFormula (2), in order to increase the rate constant k_(h*→g), it ispreferable that an emission spectrum of a host molecule (here, afluorescent spectrum because energy transfer from a singlet excitedstate is discussed) largely overlap with an absorption spectrum of aguest molecule (absorption corresponding to transition from a singletground state to a singlet excited state).

The above description suggests that the energy transfer efficiency in(2-1) can be optimized by making the fluorescent spectrum of the hostmaterial overlap with the absorption band of the substance exhibitingTADF, i.e., the guest material, which is on the longest wavelength side.

(Energy Transfer Efficiency in (2-2))

The energy transfer process in (2-2) is considered. In the process inFormula (2-1), energy transfer by Dexter mechanism (Formula (2)) ispossible. Energy transfer by Förster mechanism is forbidden in thiscase; therefore, a description thereof is omitted. In the energytransfer by Dexter mechanism, in order to increase the rate constantk_(h*→g), it is preferable as described above that an emission spectrumof a host molecule (here, a phosphorescent spectrum because energytransfer from a triplet excited state is discussed) largely overlap withan absorption spectrum of a guest molecule (absorption corresponding todirect transition from a singlet ground state to a triplet excitedstate). Note that in this structure, the guest material is a fluorescentcompound and the host material is also generally a fluorescent compound,and therefore, the spectra thereof are unlikely to be observed at roomtemperature. In such a case, a phosphorescent spectrum and an absorptionspectrum estimated by molecular orbital calculation or the like can beused. In some cases, a phosphorescent spectrum may be observed at verylow temperature (in a liquid nitrogen atmosphere or a liquid heliumatmosphere).

Note that the host molecule is generally a fluorescent compound; thus,phosphorescence lifetime (τ) is a millisecond or longer which isextremely long (i.e., k_(r)+k_(n) is low). This is because thetransition from the triplet excited state to the ground state (singlet)is a forbidden transition. Formula (3) shows that this is favorable toenergy transfer efficiency Φ_(ET).

The above description also suggests that energy transfer from the hostmolecule to the guest molecule, i.e., the processes in Formulae (2-1)and (2-2), is generally likely to occur as long as the fluorescentspectrum of the host molecule overlaps with the absorption spectrumcorresponding to the transition of the guest molecule from the singletground state to the singlet excited state, and the phosphorescentspectrum (estimated) of the host material overlaps with the absorptionspectrum (estimated) corresponding to the direct transition of the guestmaterial from the singlet ground state to the triplet excited state.

However, as for the substance exhibiting thermally activated delayedfluorescence (TADF material), especially a substance exhibitingthermally activated delayed fluorescence with high efficiency atrelatively low temperature, the S₁ level and the T₁ level are close toeach other. In other words, the absorption spectrum corresponding to thetransition of the guest molecule from the singlet ground state to thesinglet excited state is very close to the absorption spectrum(estimated) corresponding to the direct transition from the singletground state to the triplet excited state. Therefore, the host moleculeshould be designed so as to have its fluorescent spectrum andphosphorescent spectrum in similar positions.

However, in general, the S₁ level differs greatly from the T₁ level (S₁level>T₁ level); therefore, the fluorescence emission wavelength alsodiffers greatly from the phosphorescence emission wavelength(fluorescence emission wavelength<phosphorescence emission wavelength).For example, 4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP), which iscommonly used as a host molecule in a light-emitting element including aphosphorescent compound, has a phosphorescent spectrum at around 500 nmand has a fluorescent spectrum at around 400 nm, which are largelydifferent by about 100 nm. This example also shows that it is extremelydifficult to design a host molecule so as to have its fluorescentspectrum in a position similar to that of its phosphorescent spectrum.

Therefore, one embodiment of the present invention provides a usefultechnique which can overcome a problem of the efficiency of the energytransfer from the host molecule in the triplet excited state to theguest molecule when the substance exhibiting thermally activated delayedfluorescence, especially a substance exhibiting thermally activateddelayed fluorescence with high efficiency at relatively low temperature,is used as a light-emitting substance. Specific embodiments thereof willbe described below.

This embodiment provides a light-emitting element in which an exciplex(an excited complex) is used as an energy donor capable of efficientlytransferring energy to a substance exhibiting thermally activateddelayed fluorescence. The exciplex is formed from two kinds ofsubstances and is characterized in that its singlet and triplet excitedstates are close to each other. Thus, by making fluorescent lightemission of the exciplex overlap with an absorption band on the longestwavelength side which corresponds to absorption by the substanceexhibiting thermally activated delayed fluorescence, i.e., an energyacceptor, in a singlet excited state (an absorption corresponding to thetransition from the singlet ground state to the singlet excited state),it becomes possible to improve the efficiency of energy transfer fromthe singlet excited state of the exciplex to the singlet excited stateof the substance exhibiting thermally activated delayed fluorescence,and it also becomes possible to make a phosphorescent spectrum(estimated) of the exciplex in the triplet excited state overlap with anabsorption (estimated) corresponding to the direct transition of thesubstance exhibiting thermally activated delayed fluorescence from thesinglet ground state to a triplet excited state.

This makes it possible to improve the efficiency of energy transfer fromthe singlet excited state of the exciplex to the singlet excited stateof the substance exhibiting thermally activated delayed fluorescence,and also makes it possible to improve the efficiency of energy transferfrom a triplet excited state of the exciplex to the triplet excitedstate of the substance exhibiting thermally activated delayedfluorescence.

The positions of the S₁ level and the T₁ level normally differ fromsubstance to substance. In the case where fluorescent substances areused as a host material and a guest material, even when a fluorescentspectrum of a host molecule overlaps with an absorption corresponding tothe transition of a guest molecule from a singlet ground state to asinglet excited state, a phosphorescent spectrum (estimated) of the hostmolecule does not necessarily overlap with an absorption (estimated)corresponding to energy transfer of the guest molecule from the singletground state to a triplet excited state. Furthermore, in many cases, itis difficult or impossible to observe a phosphorescent spectrum of afluorescent substance and an absorption corresponding to energy transferof a fluorescent substance from a singlet ground state to a tripletexcited state. Therefore, it is difficult to determine whether or notthey overlap with each other.

Meanwhile, as described above, the S₁ level and the T₁ level of thesubstance exhibiting thermally activated delayed fluorescence are closeto each other, and those of the exciplex are close to each other.Therefore, by making the absorption corresponding to the transition ofthe substance exhibiting thermally activated delayed fluorescence fromthe singlet ground state to the singlet excited state overlap with thefluorescent spectrum of the exciplex, it becomes possible to make theabsorption (estimated) corresponding to the direct transition of thesubstance exhibiting thermally activated delayed fluorescence from thesinglet ground state to the triplet excited state overlap with thephosphorescent spectrum (estimated) of the exciplex.

In a light-emitting element having the above structure, energy transferoccurs efficiently as illustrated in FIG. 2. FIG. 2 shows that alight-emitting layer 113 is provided between an electrode 10 and anelectrode 11. There may be a given layer between each electrode and thelight-emitting layer 113. Energy transfer occurs from a singlet excitedstate Se of an exciplex 113Ec to a singlet excited state Sa of alight-emitting substance 113D, and energy transfer occurs from a tripletexcited state Te of the exciplex 113Ec to a triplet excited state Ta ofthe light-emitting substance 113D. Then, the light-emitting substance113D in the triplet excited state undergoes reverse intersystem crossingto its singlet excited state, and light emission occurs from thelight-emitting substance 113D in the singlet excited state Sa. In thelight-emitting element of this embodiment, each of these energytransfers occurs favorably; thus, the light-emitting element can havehigh emission efficiency.

FIGS. 1A and 1B are schematic diagrams of the light-emitting element ofthis embodiment. FIG. 1A is a diagram of the light-emitting element, andFIG. 1B is an enlarged diagram of only a light-emitting layer.

The light-emitting element includes an EL layer 103 between a pair ofelectrodes, a first electrode 101 and a second electrode 102, and the ELlayer 103 contains an organic compound as a light-emitting substance. Inaddition, the EL layer includes a light-emitting layer 113, and thelight-emitting substance is contained at least in the light-emittinglayer 113. There is no limitation on layers other than thelight-emitting layer, and any layer may be used as the other layers. Atypical stacked-layer structure includes a hole-injection layer 111, ahole-transport layer 112, the light-emitting layer 113, anelectron-transport layer 114, an electron-injection layer 115, and thelike. Besides, a carrier-blocking layer or the like may be provided, ora plurality of light-emitting layers may be provided.

The light-emitting layer 113 contains a first organic compound 113H, asecond organic compound 113A, and a light-emitting substance 113D asillustrated in FIG. 1B. In the light-emitting element of thisembodiment, two kinds of materials, the first organic compound 113H andthe second organic compound 113A, form a host material. Note that thisdoes not exclude the possibility that the light-emitting layer 113 inthe light-emitting element of this embodiment contains anothersubstance.

A combination of the first organic compound 113H and the second organiccompound 113A forms an exciplex. The exciplex is in a state where the S₁level and the T₁ level thereof are close to each other, and it isparticularly preferable that the combination form an exciplex having anenergy difference of 0 eV to 0.2 eV between the S₁ level and the T₁level.

The light-emitting substance 113D exhibits thermally activated delayedfluorescence, and preferably exhibits thermally activated delayedfluorescence with high efficiency at relatively low temperature (forexample, 100° C. or lower). Specifically, a substance having an energydifference of 0 eV to 0.2 eV between the S₁ level and the T₁ level ispreferable. Here, the term “delayed fluorescence” refers to lightemission exhibited by a certain substance which has the same spectrum asnormal fluorescence and has an extremely long lifetime. The lifetime is10⁻⁶ seconds or longer, preferably 10⁻³ seconds or longer.

Furthermore, the combination of the exciplex and the substanceexhibiting thermally activated delayed fluorescence has theabove-described relationship. That is, the combination makes afluorescent light emission spectrum of the exciplex overlap with anabsorption band of the substance exhibiting thermally activated delayedfluorescence which is on the longest wavelength side. Accordingly,energy is efficiently transferred from the singlet excited state of theexciplex to the singlet excited state of the substance exhibitingthermally activated delayed fluorescence.

In each of the exciplex and the substance exhibiting thermally activateddelayed fluorescence, the S₁ level and the T₁ level are close to eachother. Thus, the above-described efficient energy transfer between thesinglet excited states can bring about an increase in efficiency ofenergy transfer between the triplet excited states.

Examples of the substance exhibiting thermally activated delayedfluorescence include a fullerene, a derivative thereof, an acridinederivative such as proflavine, and eosin.

Other examples of the substance exhibiting thermally activated delayedfluorescence include a metal-containing porphyrin, such as a porphyrincontaining magnesium (Mg), zinc (Zn), cadmium (Cd), tin (Sn), platinum(Pt), indium (In), or palladium (Pd). Examples of the metal-containingporphyrin include a protoporphyrin-tin fluoride complex (SnF₂(ProtoIX)), a mesoporphyrin-tin fluoride complex (SnF₂(Meso IX)), ahematoporphyrin-tin fluoride complex (SnF₂(Hemato IX)), a coproporphyrintetramethyl ester-tin fluoride complex (SnF₂(Copro III-4Me)), anoctaethylporphyrin-tin fluoride complex (SnF₂(OEP)), anetioporphyrin-tin fluoride complex (SnF₂(Etio I)), and anoctaethylporphyrin-platinum chloride complex (PtCl₂(OEP)), which areshown in the following structural formulae.

Alternatively, a heterocyclic compound having a π-electron richheteroaromatic ring and a π-electron deficient heteroaromatic ring, suchas2-(biphenyl-4-yl)-4,6-bis(12-phenylindolo[2,3-a]carbazol-11-yl)-1,3,5-triazine(PIC-TRZ) shown in the following structural formula, can be used as thesubstance exhibiting thermally activated delayed fluorescence. Theheterocyclic compound is preferably used because of the π-electron richheteroaromatic ring and the π-electron deficient heteroaromatic ring,for which the electron-transport property and the hole-transportproperty are high. Note that a substance in which the π-electron richheteroaromatic ring is directly bonded to the π-electron deficientheteroaromatic ring is particularly preferably used because the donorproperty of the π-electron rich heteroaromatic ring and the acceptorproperty of the π-electron deficient heteroaromatic ring are bothincreased and the energy difference between the S₁ level and the T₁level becomes small.

As the combination of the first organic compound 113H and the secondorganic compound 113A, any combination can be used as long as it canform an exciplex, and known carrier-transport materials can be used. Forefficient formation of an exciplex, a combination of a compound whicheasily accepts electrons (a compound having an electron-trappingproperty) and a compound which easily accepts holes (a compound having ahole-trapping property) is preferable as the combination of the firstorganic compound and the second organic compound.

As the compound which easily accepts electrons, a π-electron deficientheteroaromatic compound, a metal complex, or the like can be used.Specific examples include a metal complex such asbis(10-hydroxybenzo[h]quinolinato)beryllium(II) (abbreviation: BeBq₂),bis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum(III)(abbreviation: BAlq), bis(8-quinolinolato)zinc(II) (abbreviation: Znq),bis[2-(2-benzoxazolyl)phenolato]zinc(II) (abbreviation: ZnPBO), orbis[2-(2-benzothiazolyl)phenolato]zinc(II) (abbreviation: ZnBTZ); aheterocyclic compound having a polyazole skeleton such as2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4triazole(abbreviation: TAZ),1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),9-[4-(5-phenyl-1,3,4-oxadiazol-2-yl)phenyl]-9H-carbazole (abbreviation:CO11), 2,2′,2″-(1,3,5-benzenetriyl)tris(1-phenyl-1H-benzimidazole)(abbreviation: TPBI), or2-[3-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: mDBTBIm-II); a heterocyclic compound having a diazineskeleton such as2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II),2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II),2-[3′-(9H-carbazol-9-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mCzBPDBq), 4,6-bis[3-(phenanthren-9-yl)phenyl]pyrimidine(abbreviation: 4,6mPnP2Pm), or4,6-bis[3-(4-dibenzothienyl)phenyl]pyrimidine (abbreviation:4,6mDBTP2Pm-II); and a heterocyclic compound having a pyridine skeletonsuch as 3,5-bis[3-(9H-carbazol-9-yl)phenyl]pyridine (abbreviation:35DCzPPy) or 1,3,5-tri[3-(3-pyridyl)phenyl]benzene (abbreviation:TmPyPB). Among the above materials, a heterocyclic compound having adiazine skeleton and a heterocyclic compound having a pyridine skeletonhave high reliability and are thus preferable. Specifically, aheterocyclic compound having a diazine (pyrimidine or pyrazine) skeletonhas a high electron-transport property to contribute to a reduction indrive voltage.

As the compound which easily accepts holes, a π-electron richheteroaromatic compound, an aromatic amine compound, or the like can befavorably used. Specific examples include a compound having an aromaticamine skeleton such as 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl(abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), 4-phenyl-3′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: mBPAFLP),4-phenyl-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine (abbreviation:PCBAlBP), 4,4′-diphenyl-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBBilBP),4-(1-naphthyl)-4′-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBANB),4,4′-di(1-naphthyl)-4″-(9-phenyl-9H-carbazol-3-yl)triphenylamine(abbreviation: PCBNBB),9,9-dimethyl-N-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]fluoren-2-amine(abbreviation: PCBAF), orN-phenyl-N-[4-(9-phenyl-9H-carbazol-3-yl)phenyl]spiro-9,9′-bifluoren-2-amine(abbreviation: PCBASF); a compound having a carbazole skeleton such as1,3-bis(N-carbazolyl)benzene (abbreviation: mCP),4,4′-di(N-carbazolyl)biphenyl (abbreviation: CBP),3,6-bis(3,5-diphenylphenyl)-9-phenylcarbazole (abbreviation: CzTP), or3,3′-bis(9-phenyl-9H-carbazole) (abbreviation: PCCP); a compound havinga thiophene skeleton such as 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation: DBT3P-II),2,8-diphenyl-4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]dibenzothiophene(abbreviation: DBTFLP-III), or4-[4-(9-phenyl-9H-fluoren-9-yl)phenyl]-6-phenyldibenzothiophene(abbreviation: DBTFLP-IV); and a compound having a furan skeleton suchas 4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzofuran) (abbreviation: DBF3P-II) or 4-{3-[3-(9-phenyl-9H-fluoren-9-yl)phenyl]phenyl}dibenzofuran(abbreviation: mmDBFFLBi-II). Among the above materials, a compoundhaving an aromatic amine skeleton and a compound having a carbazoleskeleton are preferable because these compounds are highly reliable andhave high hole-transport properties to contribute to a reduction indrive voltage.

The first organic compound and the second organic compound are notlimited to these examples, as long as they can transport carriers, thecombination can form an exciplex, and light emission of the exciplexoverlaps with an absorption band on the longest wavelength side in anabsorption spectrum of a light-emitting substance (an absorptioncorresponding to the transition of the light-emitting substance from thesinglet ground state to the singlet excited state), and other knownmaterials may be used.

Note that in the case where a compound which easily accepts electronsand a compound which easily accepts holes are used as the first organiccompound and the second organic compound, carrier balance can becontrolled by the mixture ratio of the compounds. Specifically, theratio of the first organic compound to the second organic compound ispreferably 1:9 to 9:1.

Here, compounds which foil an exciplex (the first organic compound 113Hand the second organic compound 113A) and the exciplex will be describedin a little more detail.

FIGS. 14A and 14B show emission spectra of individual substances andemission spectra of exciplexes. Note that in the figures, a compound 1is 2-[4-(dibenzothiophen-4-yl)phenyl]-1-phenyl-1H-benzimidazole(abbreviation: DBTBIm-II); a compound 2 is2-[3-(dibenzothiophen-4-yl)phenyl]dibenzo[f,h]quinoxaline (abbreviation:2mDBTPDBq-II); a compound 3 is4,4′,4″-tris[N-(1-naphthyl)-N-phenylamino]triphenylamine (abbreviation:1′-TNATA); a compound 4 is2,7-bis[N-(4-diphenylaminophenyl)-N-phenylamino]-spiro-9,9′-bifluorene(abbreviation: DPA2SF); an exciplex 1 is an exciplex of the compound 1and the compound 3; an exciplex 2 is an exciplex of the compound 2 andthe compound 3; an exciplex 3 is an exciplex of the compound 2 and4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB); andan exciplex 4 is an exciplex of the compound 2 and the compound 4.

Structural formulae of the compounds are shown below.

FIG. 14A shows emission spectra of the exciplexes 1 and 2 and thecompounds 1 to 3. The spectrum of the exciplex 1 is the result ofmeasuring light emission of a material based on the compound 1 to whicha slight amount of compound 3 is added, and the spectrum of the exciplex2 is the result of measuring light emission of a material based on thecompound 2 to which a slight amount of compound 3 is added. That is, ina sample used for measurement of the exciplex 1, one of the compounds 1and 3 corresponds to the first organic compound 113H, and the othercorresponds to the second organic compound 113A. In a sample used formeasurement of the exciplex 2, one of the compounds 2 and 3 correspondsto the first organic compound 113H, and the other corresponds to thesecond organic compound 113A.

As can be seen from FIG. 14A, there is a difference of 100 nm or morebetween light emission of the exciplex 1 and light emission of theexciplex 2 even though both materials contain the compound 3 as aslight-amount component. This means that the emission wavelength of anexciplex can be easily adjusted by changing a base substance.

Note that the peak emission wavelength of the exciplex 1 is about 520nm; thus, a host material containing the compound 1 and the compound 3can be suitably used as a host material for materials exhibitingblue-green to orange thermally activated delayed fluorescence.

The peak emission wavelength of the exciplex 2 is about 610 nm; thus, ahost material containing the compound 2 and the compound 3 can besuitably used as a host material for materials exhibiting red thermallyactivated delayed fluorescence.

FIG. 14B shows emission spectra of the exciplexes 3 and 4 and thecompounds 2 and 4. The spectrum of the exciplex 3 is the result ofmeasuring light emission of a material based on the compound 2 to whicha slight amount of NPB is added, and the spectrum of the exciplex 4 isthe result of measuring light emission of a material based on thecompound 2 to which a slight amount of the compound 4 is added. That is,in a sample used for measurement of the exciplex 3, one of the compound2 and NPB corresponds to the first organic compound 113H, and the othercorresponds to the second organic compound 113A. In a sample used formeasurement of the exciplex 4, one of the compounds 2 and 4 correspondsto the first organic compound 113H, and the other corresponds to thesecond organic compound 113A.

As can be seen from FIG. 14B, there is a difference of about 100 nmbetween light emission of the exciplex 3 and light emission of theexciplex 4 even though both materials contain the same base material.This means that the emission wavelength of an exciplex can be easilyadjusted by changing a substance that is a slight-amount component.

Note that the peak emission wavelength of the exciplex 3 is about 520nm; thus, a host material containing the compound 2 and NPB can besuitably used as a host material for materials exhibiting blue-green toorange thermally activated delayed fluorescence.

The peak emission wavelength of the exciplex 4 is about 580 nm; thus, ahost material containing the compound 2 and the compound 4 can besuitably used as a host material for materials exhibiting orange to redthermally activated delayed fluorescence. Note that in order to achievefavorable energy transfer, it is preferable that the difference inequivalent energy value between a peak wavelength in an absorption band,on the lowest energy side, of the substance exhibiting thermallyactivated delayed fluorescence and a peak wavelength of light emissionof the exciplex be 0.2 eV or less.

The light-emitting element having the above structure has highefficiency in energy transfer to the substance exhibiting thermallyactivated delayed fluorescence and has high emission efficiency.

Embodiment 2

In this embodiment, a detailed example of the structure of thelight-emitting element described in Embodiment 1 will be described belowwith reference to FIGS. 1A and 1B.

A light-emitting element in this embodiment includes, between a pair ofelectrodes, an EL layer including a plurality of layers. In thisembodiment, the light-emitting element includes the first electrode 101,the second electrode 102, and the EL layer 103 which is provided betweenthe first electrode 101 and the second electrode 102. Note that thefollowing description in this embodiment is made on the assumption thatthe first electrode 101 functions as an anode and that the secondelectrode 102 functions as a cathode. In other words, when a voltage isapplied between the first electrode 101 and the second electrode 102 sothat the potential of the first electrode 101 is higher than that of thesecond electrode 102, light emission can be obtained.

Since the first electrode 101 functions as the anode, the firstelectrode 101 is preferably formed using any of metals, alloys,electrically conductive compounds with a high work function(specifically, a work function of 4.0 eV or more), mixtures thereof, andthe like. Specifically, for example, indium oxide-tin oxide (ITO: indiumtin oxide), indium oxide-tin oxide containing silicon or silicon oxide,indium oxide-zinc oxide, indium oxide containing tungsten oxide and zincoxide (IWZO), and the like can be given. Films of these electricallyconductive metal oxides are usually formed by a sputtering method butmay be formed by application of a sol-gel method or the like. In anexample of the formation method, indium oxide-zinc oxide is deposited bya sputtering method using a target obtained by adding 1 wt % to 20 wt %of zinc oxide to indium oxide. Further, a film of indium oxidecontaining tungsten oxide and zinc oxide (IWZO) can be formed by asputtering method using a target in which tungsten oxide and zinc oxideare added to indium oxide at 0.5 wt % to 5 wt % and 0.1 wt % to 1 wt %,respectively. Besides, gold (Au), platinum (Pt), nickel (Ni), tungsten(W), chromium (Cr), molybdenum (Mo), iron (Fe), cobalt (Co), copper(Cu), palladium (Pd), nitrides of metal materials (e.g., titaniumnitride), and the like can be given. Graphene can also be used. Notethat when a composite material described later is used for a layer whichis in contact with the first electrode 101 in the EL layer 103, anelectrode material can be selected regardless of its work function.

There is no particular limitation on the stacked-layer structure of theEL layer 103 as long as the light-emitting layer 113 has the structuredescribed in Embodiment 1. For example, the EL layer 103 can be formedby combining a hole-injection layer, a hole-transport layer, thelight-emitting layer, an electron-transport layer, an electron-injectionlayer, a carrier-blocking layer, an intermediate layer, and the like asappropriate. In this embodiment, the EL layer 103 has a structure inwhich a hole-injection layer 111, a hole-transport layer 112, alight-emitting layer 113, an electron-transport layer 114, and anelectron-injection layer 115 are stacked in this order over the firstelectrode 101. Specific examples of materials used for each layer aregiven below.

The hole-injection layer 111 is a layer containing a substance having ahigh hole-injection property. Molybdenum oxide, vanadium oxide,ruthenium oxide, tungsten oxide, manganese oxide, or the like can beused. Alternatively, the hole-injection layer 111 can be formed using aphthalocyanine-based compound such as phthalocyanine (abbreviation:H₂Pc) or copper phthalocyanine (abbreviation: CuPc), an aromatic aminecompound such as4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl (abbreviation:DPAB) orN,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD), a high molecular compound such aspoly(ethylenedioxythiophene)/poly(styrenesulfonic acid) (abbreviation:PEDOT/PSS), or the like.

Alternatively, a composite material in which a substance having ahole-transport property contains a substance having an acceptor propertycan be used for the hole-injection layer 111. Note that the use of sucha substance having a hole-transport property which contains a substancehaving an acceptor property enables selection of a material used to forman electrode regardless of its work function. In other words, besides amaterial having a high work function, a material having a low workfunction can also be used for the first electrode 101. As the substancehaving an acceptor property,7,7,8,8-tetracyano-2,3,5,6-tetrafluoroquinodimethane (abbreviation:F₄-TCNQ), chloranil, and the like can be given. In addition, transitionmetal oxides can be given. Oxides of the metals that belong to Groups 4to 8 of the periodic table can be given. Specifically, vanadium oxide,niobium oxide, tantalum oxide, chromium oxide, molybdenum oxide,tungsten oxide, manganese oxide, and rhenium oxide are preferable inthat their electron-accepting property is high. Among these oxides,molybdenum oxide is particularly preferable in that it is stable in theair, has a low hygroscopic property, and is easy to handle.

As the substance having a hole-transport property which is used for thecomposite material, any of a variety of organic compounds such asaromatic amine compounds, carbazole derivatives, aromatic hydrocarbons,and high molecular compounds (e.g., oligomers, dendrimers, or polymers)can be used. Note that the organic compound used for the compositematerial is preferably an organic compound having a high hole-transportproperty. Specifically, a substance having a hole mobility of 10⁻⁶cm²/Vs or more is preferably used. Organic compounds that can be used asthe substance having a hole-transport property in the composite materialare specifically given below.

Examples of the aromatic amine compounds areN,N′-di(p-tolyl)-N,N′-diphenyl-p-phenylenediamine (abbreviation:DTDPPA), 4,4′-bis[N-(4-diphenylaminophenyl)-N-phenylamino]biphenyl(abbreviation: DPAB),N,N′-bis{4-[bis(3-methylphenyl)amino]phenyl}-N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine(abbreviation: DNTPD),1,3,5-tris[N-(4-diphenylaminophenyl)-N-phenylamino]benzene(abbreviation: DPA3B), and the like.

Specific examples of the carbazole derivatives that can be used for thecomposite material are3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1),3,6-bis[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA2),3-[N-(1-naphthyl)-N-(9-phenylcarbazol-3-yl)amino]-9-phenylcarbazole(abbreviation: PCzPCN1), and the like.

Other examples of the carbazole derivatives that can be used for thecomposite material are 4,4′-di(N-carbazolyl)biphenyl (abbreviation:CBP), 1,3,5-tris[4-(N-carbazolyl)phenyl]benzene (abbreviation: TCPB),9-[4-(10-phenyl-9-anthracenyl)phenyl]-9H-carbazole (abbreviation: CzPA),1,4-bis[4-(N-carbazolyl)phenyl]-2,3,5,6-tetraphenylbenzene, and thelike.

Examples of the aromatic hydrocarbons that can be used for the compositematerial are 2-tert-butyl-9,10-di(2-naphthyl)anthracene (abbreviation:t-BuDNA), 2-tert-butyl-9,10-di(1-naphthyl)anthracene,9,10-bis(3,5-diphenylphenyl)anthracene (abbreviation: DPPA),2-tert-butyl-9,10-bis(4-phenylphenyl)anthracene (abbreviation: t-BuDBA),9,10-di(2-naphthyl)anthracene (abbreviation: DNA),9,10-diphenylanthracene (abbreviation: DPAnth), 2-tert-butylanthracene(abbreviation: t-BuAnth), 9,10-bis(4-methyl-1-naphthyl)anthracene(abbreviation: DMNA),2-tert-butyl-9,10-bis[2-(1-naphthyl)phenyl]anthracene,9,10-bis[2-(1-naphthyl)phenyl]anthracene,2,3,6,7-tetramethyl-9,10-di(1-naphthyl)anthracene,2,3,6,7-tetramethyl-9,10-di(2-naphthyl)anthracene, 9,9′-bianthryl,10,10′-diphenyl-9,9′-bianthryl,10,10′-bis(2-phenylphenyl)-9,9′-bianthryl,10,10′-bis[(2,3,4,5,6-pentaphenyl)phenyl]-9,9′-bianthryl, anthracene,tetracene, rubrene, perylene, 2,5,8,11-tetra(tert-butyl)perylene, andthe like. Besides, pentacene, coronene, or the like can also be used.The aromatic hydrocarbon which has a hole mobility of 1×10⁻⁶ cm²/Vs ormore and which has 14 to 42 carbon atoms is particularly preferable.

Note that the aromatic hydrocarbons that can be used for the compositematerial may have a vinyl skeleton. Examples of the aromatic hydrocarbonhaving a vinyl group are 4,4′-bis(2,2-diphenylvinyl)biphenyl(abbreviation: DPVBi), 9,10-bis[4-(2,2-diphenylvinyl)phenyl]anthracene(abbreviation: DPVPA), and the like.

A polymeric compound such as poly(N-vinylcarbazole) (abbreviation: PVK),poly(4-vinyltriphenylamine) (abbreviation: PVTPA),poly[N-(4-{N′-[4-(4-diphenylamino)phenyl]phenyl-N′-phenylamino}phenyl)methacrylamide](abbreviation: PTPDMA), orpoly[N,N′-bis(4-butylphenyl)-N,N′-bis(phenyl)benzidine](abbreviation:poly-TPD) can also be used.

By providing a hole-injection layer, a high hole-injection property canbe achieved to allow a light-emitting element to be driven at a lowvoltage.

The hole-transport layer 112 is a layer that contains a substance havinga hole-transport property. Examples of the substance having ahole-transport property are aromatic amine compounds such as4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (abbreviation: NPB),N,N′-bis(3-methylphenyl)-N,N′-diphenyl-[1,1′-biphenyl]-4,4′-diamine(abbreviation: TPD), 4,4′,4″-tris(N,N-diphenylamino)triphenylamine(abbreviation: TDATA),4,4′,4″-tris[N-(3-methylphenyl)-N-phenylamino]triphenylamine(abbreviation: MTDATA),4,4′-bis[N-(spiro-9,9′-bifluoren-2-yl)-N-phenylamino]biphenyl(abbreviation: BSPB), 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP), and the like. The substances mentioned here havehigh hole-transport properties and are mainly ones that have a holemobility of 10⁻⁶ cm²/Vs or more. An organic compound given as an exampleof the substance having a hole-transport property in the compositematerial described above can also be used for the hole-transport layer112. A polymeric compound such as poly(N-vinylcarbazole) (abbreviation:PVK) or poly(4-vinyltriphenylamine) (abbreviation: PVTPA) can also beused. Note that the layer that contains a substance having ahole-transport property is not limited to a single layer, and may be astack of two or more layers including any of the above substances.

The light-emitting layer 113 contains a light-emitting substance, afirst organic compound, and a second organic compound. Since thelight-emitting layer 113 has the structure described in Embodiment 1,the light-emitting element in this embodiment can have extremely highemission efficiency. Embodiment 1 is to be referred to for thecomponents of the light-emitting layer 113.

The light-emitting layer 113 having the above-described structure can beformed by co-evaporation by a vacuum evaporation method, or an inkjetmethod, a spin coating method, a dip coating method, or the like using amixed solution.

The electron-transport layer 114 is a layer containing a substancehaving an electron-transport property. For example, a layer containing ametal complex having a quinoline skeleton or a benzoquinoline skeleton,such as tris(8-quinolinolato)aluminum (abbreviation: Alq),tris(4-methyl-8-quinolinolato)aluminum (abbreviation: Almq₃),bis(10-hydroxybenzo[h]quinolinato)beryllium (abbreviation: BeBq₂), orbis(2-methyl-8-quinolinolato)(4-phenylphenolato)aluminum (abbreviation:BAlq), or the like can be used. Alternatively, a metal complex having anoxazole-based or thiazole-based ligand, such asbis[2-(2-hydroxyphenyl)benzoxazolato]zinc (abbreviation: Zn(BOX)₂) orbis[2-(2-hydroxyphenyl)benzothiazolato]zinc (abbreviation: Zn(BTZ)₂), orthe like can be used. Besides the metal complexes,2-(4-biphenylyl)-5-(4-tent-butylphenyl)-1,3,4-oxadiazole (abbreviation:PBD), 1,3-bis[5-(p-tert-butylphenyl)-1,3,4-oxadiazol-2-yl]benzene(abbreviation: OXD-7),3-(4-biphenylyl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole(abbreviation: TAZ), bathophenanthroline (abbreviation: BPhen),bathocuproine (abbreviation: BCP), or the like can also be used. Thesubstances mentioned here have high electron-transport properties andare mainly ones that have an electron mobility of 10⁻⁶ cm²/Vs or more.Note that any of the above-described host materials havingelectron-transport properties may be used for the electron-transportlayer 114.

The electron-transport layer 114 is not limited to a single layer, andmay be a stack of two or more layers containing any of the abovesubstances.

Between the electron-transport layer and the light-emitting layer, alayer that controls transport of electron carriers may be provided. Thisis a layer formed by addition of a small amount of a substance having ahigh electron-trapping property to the aforementioned material having ahigh electron-transport property, and the layer is capable of adjustingcarrier balance by retarding transport of electron carriers. Such astructure is very effective in preventing a problem (such as a reductionin element lifetime) caused when electrons pass through thelight-emitting layer.

In addition, the electron-injection layer 115 may be provided in contactwith the second electrode 102. For the electron-injection layer 115, analkali metal, an alkaline earth metal, or a compound thereof, such aslithium fluoride (LiF), cesium fluoride (CsF), or calcium fluoride(CaF₂), can be used. For example, a layer that is formed using asubstance having an electron-transport property and contains an alkalimetal, an alkaline earth metal, or a compound thereof can be used. Notethat a layer that is formed using a substance having anelectron-transport property and contains an alkali metal or an alkalineearth metal is preferably used as the electron-injection layer 115, inwhich case electron injection from the second electrode 102 isefficiently performed.

For the second electrode 102, any of metals, alloys, electricallyconductive compounds, and mixtures thereof which have a low workfunction (specifically, a work function of 3.8 eV or less) or the likecan be used. Specific examples of such a cathode material are elementsbelonging to Groups 1 and 2 of the periodic table, such as alkali metals(e.g., lithium (Li) and cesium (Cs)), magnesium (Mg), calcium (Ca), andstrontium (Sr), alloys thereof (e.g., MgAg and AlLi), rare earth metalssuch as europium (Eu) and ytterbium (Yb), alloys thereof, and the like.However, when the electron-injection layer is provided between thesecond electrode 102 and the electron-transport layer, for the secondelectrode 102, any of a variety of conductive materials such as Al, Ag,ITO, or indium oxide-tin oxide containing silicon or silicon oxide canbe used regardless of the work function. Films of these electricallyconductive materials can be formed by a sputtering method, an inkjetmethod, a spin coating method, or the like.

Any of a variety of methods can be used to form the EL layer 103regardless whether it is a dry process or a wet process. For example, avacuum evaporation method, an inkjet method, a spin coating method, orthe like may be used. Different formation methods may be used for theelectrodes or the layers.

In addition, the electrode may be formed by a wet method using a sol-gelmethod, or by a wet method using paste of a metal material.Alternatively, the electrode may be formed by a dry method such as asputtering method or a vacuum evaporation method.

In the light-emitting element having the above-described structure,current flows due to a potential difference applied between the firstelectrode 101 and the second electrode 102, and holes and electronsrecombine in the light-emitting layer 113 which contains alight-emitting substance, so that light is emitted. That is, alight-emitting region is formed in the light-emitting layer 113.

Light emission is extracted out through one or both of the firstelectrode 101 and the second electrode 102. Therefore, one or both ofthe first electrode 101 and the second electrode 102 arelight-transmitting electrodes. In the case where only the firstelectrode 101 is a light-transmitting electrode, light emission isextracted through the first electrode 101. In the case where only thesecond electrode 102 is a light-transmitting electrode, light emissionis extracted through the second electrode 102.

In the case where both the first electrode 101 and the second electrode102 are light-transmitting electrodes, light emission is extractedthrough the first electrode 101 and the second electrode 102.

The structure of the layers provided between the first electrode 101 andthe second electrode 102 is not limited to the above-describedstructure. Preferably, a light-emitting region where holes and electronsrecombine is positioned away from the first electrode 101 and the secondelectrode 102 so that quenching due to the proximity of thelight-emitting region and a metal used for electrodes andcarrier-injection layers can be prevented.

Further, in order that transfer of energy from an exciton generated inthe light-emitting layer can be suppressed, preferably, thehole-transport layer and the electron-transport layer which are incontact with the light-emitting layer 113, particularly acarrier-transport layer in contact with a side closer to thelight-emitting region in the light-emitting layer 113, are formed usinga substance having a wider band gap than the light-emitting substance ofthe light-emitting layer or the emission center substance included inthe light-emitting layer.

The light-emitting element in this embodiment is provided over asubstrate of glass, plastic, a metal, or the like. Note that as asubstrate which transmits light from the light-emitting element, asubstrate having a high visible light transmitting property is used. Asthe way of stacking layers over the substrate, layers may besequentially stacked from the first electrode 101 side or sequentiallystacked from the second electrode 102 side. In a light-emitting device,although one light-emitting element may be formed over one substrate, aplurality of light-emitting elements may be formed over one substrate.With a plurality of light-emitting elements as described above formedover one substrate, a lighting device in which elements are separated ora passive-matrix light-emitting device can be manufactured. Alight-emitting element may be formed over an electrode electricallyconnected to a thin film transistor (TFT), for example, which is formedover a substrate of glass, plastic, or the like, so that an activematrix light-emitting device in which the TFT controls the drive of thelight-emitting element can be manufactured. Note that there is noparticular limitation on the structure of the TFT, which may be astaggered TFT or an inverted staggered TFT. In addition, crystallinityof a semiconductor used for the TFT is not particularly limited either;an amorphous semiconductor or a crystalline semiconductor may be used.In addition, a driver circuit formed in a TFT substrate may be formedwith an n-type TFT and a p-type TFT, or with either an n-type TFT or ap-type TFT.

Note that this embodiment can be combined with any of the otherembodiments as appropriate.

Embodiment 3

In this embodiment, a light-emitting device including the light-emittingelement described in Embodiment 1 or 2 will be described.

In this embodiment, the light-emitting device manufactured using thelight-emitting element described in Embodiment 1 or 2 is described withreference to FIGS. 3A and 3B. Note that FIG. 3A is a top view of thelight-emitting device and FIG. 3B is a cross-sectional view taken alongthe lines A-B and C-D in FIG. 3A. This light-emitting device includes adriver circuit portion (source line driver circuit) 601, a pixel portion602, and a driver circuit portion (gate line driver circuit) 603, whichare to control light emission of a light-emitting element 618 andillustrated with dotted lines. Moreover, a reference numeral 604 denotesa sealing substrate; 625, a drying agent; 605, a sealing material; and607, a space surrounded by the sealing material 605.

Reference numeral 608 denotes a wiring for transmitting signals to beinput to the source line driver circuit 601 and the gate line drivercircuit 603 and receiving signals such as a video signal, a clocksignal, a start signal, and a reset signal from an FPC (flexible printedcircuit) 609 serving as an external input terminal. Although only theFPC is illustrated here, a printed wiring board (PWB) may be attached tothe FPC. The light-emitting device in the present specificationincludes, in its category, not only the light-emitting device itself butalso the light-emitting device provided with the FPC or the PWB.

The driver circuit portion and the pixel portion are formed over anelement substrate 610; FIG. 3B shows the source line driver circuit 601,which is a driver circuit portion, and one of the pixels in the pixelportion 602.

As the source line driver circuit 601, a CMOS circuit in which ann-channel TFT 623 and a p-channel TFT 624 are combined is formed. Inaddition, the driver circuit may be formed with any of a variety ofcircuits such as a CMOS circuit, a PMOS circuit, or an NMOS circuit.Although a driver integrated type in which the driver circuit is faultedover the substrate is illustrated in this embodiment, the driver circuitis not necessarily formed over the substrate, and the driver circuit canbe formed outside, not over the substrate.

The pixel portion 602 includes a plurality of pixels including aswitching TFT 611, a current controlling TFT 612, and a first electrode613 electrically connected to a drain of the current controlling TFT612. Note that to cover an end portion of the first electrode 613, aninsulator 614 is formed, for which a positive photosensitive acrylicresin film is used here.

In order to improve coverage, the insulator 614 is formed to have acurved surface with curvature at its upper or lower end portion. Forexample, in the case where positive photosensitive acrylic is used for amaterial of the insulator 614, only the upper end portion of theinsulator 614 preferably has a curved surface with a curvature radius(0.2 μm to 3 μm). As the insulator 614, either a negative photosensitiveresin or a positive photosensitive resin can be used.

An EL layer 616 and a second electrode 617 are formed over the firstelectrode 613. Here, as a material used for the first electrode 613functioning as an anode, a material having a high work function ispreferably used. For example, a single-layer film of an ITO film, anindium tin oxide film containing silicon, an indium oxide filmcontaining zinc oxide at 2 wt % to 20 wt %, a titanium nitride film, achromium film, a tungsten film, a Zn film, a Pt film, or the like, astack of a titanium nitride film and a film containing aluminum as itsmain component, a stack of three layers of a titanium nitride film, afilm containing aluminum as its main component, and a titanium nitridefilm, or the like can be used. The stacked-layer structure enables lowwiring resistance, favorable ohmic contact, and a function as an anode.

In addition, the EL layer 616 is formed by any of a variety of methodssuch as an evaporation method using an evaporation mask, an inkjetmethod, and a spin coating method. The EL layer 616 has the structuredescribed in Embodiment 1 or 2. Further, for another material includedin the EL layer 616, any of low molecular-weight compounds and polymericcompounds (including oligomers and dendrimers) may be used.

As a material used for the second electrode 617, which is formed overthe EL layer 616 and functions as a cathode, a material having a lowwork function (e.g., Al, Mg, Li, Ca, or an alloy or a compound thereof,such as MgAg, MgIn, or AlLi) is preferably used. In the case where lightgenerated in the EL layer 616 is transmitted through the secondelectrode 617, a stack of a thin metal film and a transparent conductivefilm (e.g., ITO, indium oxide containing zinc oxide at 2 wt % to 20 wt%, indium tin oxide containing silicon, or zinc oxide (ZnO)) ispreferably used for the second electrode 617.

Note that the light-emitting element 618 is formed with the firstelectrode 613, the EL layer 616, and the second electrode 617. Thelight-emitting element has the structure described in Embodiment 2. Inthe light-emitting device of this embodiment, the pixel portion 602,which includes a plurality of light-emitting elements, may include boththe light-emitting element described in Embodiment 1 or 2 and alight-emitting element having a different structure.

Further, the sealing substrate 604 is attached to the element substrate610 with the sealing material 605, so that the light-emitting element618 is provided in the space 607 surrounded by the element substrate610, the sealing substrate 604, and the sealing material 605. The space607 may be filled with filler, or may be filled with an inert gas (suchas nitrogen or argon), or the sealing material 605. It is preferablethat the sealing substrate be provided with a recessed portion and thedrying agent 625 be provided in the recessed portion, in which casedeterioration due to influence of moisture can be suppressed.

An epoxy-based resin or glass frit is preferably used for the sealingmaterial 605. It is preferable that such a material do not transmitmoisture or oxygen as much as possible. As the sealing substrate 604, aglass substrate, a quartz substrate, or a plastic substrate fanned offiberglass reinforced plastic (FRP), poly(vinyl fluoride) (PVF),polyester, acrylic, or the like can be used.

As described above, the light-emitting device which uses thelight-emitting element described in Embodiment 1 or 2 can be obtained.

The light-emitting device in this embodiment is fabricated using thelight-emitting element described in Embodiment 1 or 2 and thus can havefavorable characteristics. Specifically, since the light-emittingelement described in Embodiment 1 or 2 has high emission efficiency, thelight-emitting device can have reduced power consumption. In addition,since the light-emitting element can be driven at low voltage, thelight-emitting device can be driven at low voltage.

Although an active matrix light-emitting device is described above inthis embodiment, application to a passive matrix light-emitting devicemay be carried out. FIGS. 4A and 4B illustrate a passive matrixlight-emitting device manufactured using the present invention. FIG. 4Ais a perspective view of the light-emitting device, and FIG. 4B is across-sectional view taken along the line X-Y in FIG. 4A. In FIGS. 4Aand 4B, over a substrate 951, an EL layer 955 is provided between anelectrode 952 and an electrode 956. An end portion of the electrode 952is covered with an insulating layer 953. In addition, a partition layer954 is provided over the insulating layer 953. The sidewalls of thepartition layer 954 are aslope such that the distance between bothsidewalls is gradually narrowed toward the surface of the substrate. Inother words, a cross section taken along the direction of the short sideof the partition layer 954 is trapezoidal, and the lower side (a sidewhich is in contact with the insulating layer 953) is shorter than theupper side (a side which is not in contact with the insulating layer953). The partition layer 954 thus provided can prevent defects in thelight-emitting element due to cross-talk or the like. The passive matrixlight-emitting device can also be driven with low power consumption byincluding the light-emitting element in Embodiment 1 or 2 which can bedriven at low voltage. The passive matrix light-emitting device can bedriven with low power consumption by including the light-emittingelement in Embodiment 1 or 2.

Further, for performing full color display, a coloring layer or a colorconversion layer may be provided in a light path through which lightfrom the light-emitting element passes to the outside of thelight-emitting device. An example of a light-emitting device in whichfull color display is achieved with the use of a coloring layer and thelike is illustrated in FIGS. 5A and 5B. In FIG. 5A, a substrate 1001, abase insulating film 1002, a gate insulating film 1003, gate electrodes1006, 1007, and 1008, a first interlayer insulating film 1020, a secondinterlayer insulating film 1021, a peripheral portion 1042, a pixelportion 1040, a driver circuit portion 1041, first electrodes 1024W,1024R, 10246 and 1024B of light-emitting elements, a partition 1025, anEL layer 1028, a second electrode 1029 of the light-emitting elements, asealing substrate 1031, a sealant 1032, and the like are illustrated.Further, coloring layers (a red coloring layer 1034R, a green coloringlayer 1034G, and a blue coloring layer 1034B) are provided on atransparent base material 1033. Further, a black layer (a black matrix)1035 may be additionally provided. The transparent base material 1033provided with the coloring layers and the black layer is fixed to thesubstrate 1001. Note that the coloring layers and the black layer arecovered with an overcoat layer 1036. In this embodiment, light emittedfrom part of the light-emitting layer does not pass through the coloringlayers, while light emitted from the other part of the light-emittinglayer passes through the coloring layers. Since light which does notpass through the coloring layers is white and light which passes throughany one of the coloring layers is red, blue, or green, a full colorimage can be displayed using pixels of the four colors.

The above-described light-emitting device is a light-emitting devicehaving a structure in which light is extracted from the substrate 1001side where the TFTs are formed (a bottom emission structure), but may bea light-emitting device having a structure in which light is extractedfrom the sealing substrate 1031 side (a top emission structure). FIG. 6is a cross-sectional view of a light-emitting device having a topemission structure. In this case, a substrate which does not transmitlight can be used as the substrate 1001. Apart from the structureillustrated in FIG. 5A, a third interlayer insulating film 1037 isformed to cover an electrode 1022. This insulating film may have aplanarization function.

The first electrodes 1024W, 1024R, 1024G, and 1024B of thelight-emitting elements each serve as an anode here, but may serve as acathode. The first electrodes are reflective electrodes. The EL layer1028 is formed to have the structure described in Embodiment 1 or 2,with which white light emission can be obtained.

The coloring layers are each provided in a light path through whichlight from the light-emitting element passes to the outside of thelight-emitting device. In the case of the light-emitting device having abottom emission structure as illustrated in FIG. 5A, the coloring layers1034R, 1034G, and 1034B can be provided on the transparent base material1033 and then fixed to the substrate 1001. The coloring layers may beprovided between the gate insulating film 1003 and the first interlayerinsulating film 1020 as illustrated in FIG. 5B. In the case of a topemission structure as illustrated in FIG. 6, sealing can be performedwith the sealing substrate 1031 on which the coloring layers (the redcoloring layer 1034R, the green coloring layer 1034G, and the bluecoloring layer 1034B) are provided. The sealing substrate 1031 may beprovided with a black layer 1036 which is positioned between pixels. Thecoloring layers (the red coloring layer 10348, the green coloring layer1034G, and the blue coloring layer 1034B) and the black layer 1036 maybe covered with an overcoat layer. Note that a light-transmittingsubstrate is used as the sealing substrate 1031.

When voltage is applied between the pair of electrodes of the thusobtained organic light-emitting element, a white light-emitting region1044W can be obtained. In addition, by using the coloring layers, a redlight-emitting region 10448, a blue light-emitting region 1044B, and agreen light-emitting region 1044G can be obtained. The light-emittingdevice in this embodiment includes the light-emitting element describedin Embodiment 1 or 2; thus, a light-emitting device with low powerconsumption can be obtained.

Further, although an example in which full color display is performedusing four colors of red, green, blue, and white is shown here, there isno particular limitation and full color display using three colors ofred, green, and blue may be performed.

This embodiment can be freely combined with any of other embodiments.

Embodiment 4

In this embodiment, an example in which the light-emitting elementdescribed in Embodiment 1 or 2 is used for a lighting device will bedescribed with reference to FIGS. 7A and 7B. FIG. 7B is a top view ofthe lighting device, and FIG. 7A is a cross-sectional view taken alongthe line e-f in FIG. 7B.

In the lighting device in this embodiment, a first electrode 401 isformed over a substrate 400 which is a support and has alight-transmitting property. The first electrode 401 corresponds to thefirst electrode 101 in Embodiment 1 or 2.

An auxiliary electrode 402 is provided over the first electrode 401.Since light emission is extracted through the first electrode 401 sidein the example given in this embodiment, the first electrode 401 isformed using a material having a light-transmitting property. Theauxiliary electrode 402 is provided in order to compensate for the lowconductivity of the material having a light-transmitting property, andhas a function of suppressing luminance unevenness in a light emissionsurface due to voltage drop caused by the high resistance of the firstelectrode 401. The auxiliary electrode 402 is formed using a materialhaving at least higher conductivity than the material of the firstelectrode 401, and is preferably formed using a material having highconductivity such as aluminum. Note that surfaces of the auxiliaryelectrode 402 other than a portion thereof in contact with the firstelectrode 401 are preferably covered with an insulating layer. This isfor suppressing light emission over the upper portion of the auxiliaryelectrode 402, which cannot be extracted, for reducing a reactivecurrent, and for suppressing a reduction in power efficiency. Note thata pad 412 for applying a voltage to a second electrode 404 may be formedat the same time as the formation of the auxiliary electrode 402.

An EL layer 403 is formed over the first electrode 401 and the auxiliaryelectrode 402. The EL layer 403 has the structure described inEmbodiment 1 or 2. Refer to the descriptions for the structure. Notethat the EL layer 403 is preferably formed to be slightly larger thanthe first electrode 401 when seen from above, in which case the EL layer403 can also serve as an insulating layer that suppresses a shortcircuit between the first electrode 401 and the second electrode 404.

The second electrode 404 is formed to cover the EL layer 403. The secondelectrode 404 corresponds to the second electrode 102 in Embodiment 1 or2 and has a similar structure. In this embodiment, it is preferable thatthe second electrode 404 be formed using a material having highreflectance because light emission is extracted through the firstelectrode 401 side. In this embodiment, the second electrode 404 isconnected to the pad 412, whereby voltage is applied.

As described above, the lighting device described in this embodimentincludes a light-emitting element including the first electrode 401, theEL layer 403, and the second electrode 404 (and the auxiliary electrode402). Since the light-emitting element is a light-emitting element withhigh emission efficiency, the lighting device in this embodiment can bea lighting device having low power consumption. Furthermore, since thelight-emitting element is a light-emitting element having highreliability, the lighting device in this embodiment can be a lightingdevice having high reliability.

The light-emitting element having the above structure is fixed to asealing substrate 407 with sealing materials 405 and 406 and sealing isperformed, whereby the lighting device is completed. It is possible touse only either the sealing material 405 or the sealing material 406. Inaddition, the inner sealing material 406 can be mixed with a desiccantwhich enables moisture to be adsorbed, increasing reliability.

When parts of the pad 412, the first electrode 401, and the auxiliaryelectrode 402 are extended to the outside of the sealing materials 405and 406, the extended parts can serve as external input terminals. An ICchip 420 mounted with a converter or the like may be provided over theexternal input terminals.

As described above, since the lighting device described in thisembodiment includes the light-emitting element described in Embodiment 1or 2 as an EL element, the lighting device can be a lighting devicehaving low power consumption. Further, the lighting device can be alighting device which can be driven at low voltage. Furthermore, thelighting device can be a lighting device having high reliability.

Embodiment 5

In this embodiment, examples of electronic devices each including thelight-emitting element described in Embodiment 1 or 2 will be described.The light-emitting element described in Embodiment 1 or 2 has highemission efficiency and reduced power consumption. As a result, theelectronic devices described in this embodiment can each include alight-emitting portion having reduced power consumption. In addition,the electronic devices can be driven at low voltage since thelight-emitting element described in Embodiment 1 or 2 can be driven atlow voltage.

Examples of the electronic device to which the above light-emittingelement is applied include television devices (also referred to as TV ortelevision receivers), monitors for computers and the like, cameras suchas digital cameras and digital video cameras, digital photo frames,mobile phones (also referred to as cell phones or mobile phone devices),portable game machines, portable information terminals, audio playbackdevices, large game machines such as pachinko machines, and the like.Specific examples of these electronic devices are given below.

FIG. 8A illustrates an example of a television device. In the televisiondevice, a display portion 7103 is incorporated in a housing 7101. Inaddition, here, the housing 7101 is supported by a stand 7105. Imagescan be displayed on the display portion 7103, and in the display portion7103, the light-emitting elements described in Embodiment 1 or 2 arearranged in a matrix. The light-emitting element can have high emissionefficiency. Further, the light-emitting elements can be driven at lowvoltage. Furthermore, the light-emitting element can have a longlifetime. Therefore, the television device including the display portion7103 which is formed using the light-emitting element can exhibitreduced power consumption. Further, the television device can be drivenat low voltage. Furthermore, the television device can have highreliability.

Operation of the television device can be performed with an operationswitch of the housing 7101 or a separate remote controller 7110. Withoperation keys 7109 of the remote controller 7110, channels and volumecan be controlled and images displayed on the display portion 7103 canbe controlled. Furthermore, the remote controller 7110 may be providedwith a display portion 7107 for displaying data output from the remotecontroller 7110.

Note that the television device is provided with a receiver, a modem,and the like. With the use of the receiver, general televisionbroadcasting can be received. Moreover, when the television device isconnected to a communication network with or without wires via themodem, one-way (from a sender to a receiver) or two-way (between asender and a receiver or between receivers) information communicationcan be performed.

FIG. 8B1 illustrates a computer, which includes a main body 7201, ahousing 7202, a display portion 7203, a keyboard 7204, an externalconnection port 7205, a pointing device 7206, and the like. Note thatthis computer is manufactured by arranging the light-emitting elementsdescribed in Embodiment 1 or 2 in a matrix in the display portion 7203.The computer illustrated in FIG. 8B1 may have a structure illustrated inFIG. 8B2. The computer illustrated in FIG. 8B2 is provided with a seconddisplay portion 7210 instead of the keyboard 7204 and the pointingdevice 7206. The second display portion 7210 has a touch screen, andinput can be performed by operation of images, which are displayed onthe second display portion 7210, with a finger or a dedicated pen. Thesecond display portion 7210 can also display images other than thedisplay for input. The display portion 7203 may also have a touchscreen. Connecting the two screens with a hinge can prevent troubles;for example, the screens can be prevented from being cracked or brokenwhile the computer is being stored or carried. The light-emittingelements can have high emission efficiency. Therefore, this computerhaving the display portion 7203 which is formed using the light-emittingelements can have reduced power consumption.

FIG. 8C illustrates a portable game machine having two housings, ahousing 7301 and a housing 7302, which are connected with a jointportion 7303 so that the portable game machine can be opened or folded.The housing 7301 incorporates a display portion 7304 including thelight-emitting elements described in Embodiment 1 or 2 and arranged in amatrix, and the housing 7302 incorporates a display portion 7305. Inaddition, the portable game machine illustrated in FIG. 8C includes aspeaker portion 7306, a recording medium insertion portion 7307, an LEDlamp 7308, input means (an operation key 7309, a connection terminal7310, a sensor 7311 (a sensor having a function of measuring or sensingforce, displacement, position, speed, acceleration, angular velocity,rotational frequency, distance, light, liquid, magnetism, temperature,chemical substance, sound, time, hardness, electric field, current,voltage, electric power, radiation, flow rate, humidity, gradient,oscillation, odor, or infrared rays), and a microphone 7312), and thelike. Needless to say, the structure of the portable game machine is notlimited to the above as long as the display portion which includes thelight-emitting elements described in Embodiment 1 or 2 and arranged in amatrix is used as either the display portion 7304 or the display portion7305, or both, and the structure can include other accessories asappropriate. The portable game machine illustrated in FIG. 8C has afunction of reading out a program or data stored in a storage medium todisplay it on the display portion, and a function of sharing informationwith another portable game machine by wireless communication. Note thatfunctions of the portable game machine illustrated in FIG. 8C are notlimited to them, and the portable game machine can have variousfunctions. Since the light-emitting elements used in the display portion7304 have high emission efficiency, the portable game machine includingthe above-described display portion 7304 can have reduced powerconsumption. Since each of the light-emitting elements used in thedisplay portion 7304 can be driven at low voltage, the portable gamemachine can also be driven at low voltage. Furthermore, since thelight-emitting elements used in the display portion 7304 each have along lifetime, the portable game machine can have high reliability.

FIG. 8D illustrates an example of a mobile phone. The mobile phone isprovided with a display portion 7402 incorporated in a housing 7401,operation buttons 7403, an external connection port 7404, a speaker7405, a microphone 7406, and the like. Note that the mobile phone hasthe display portion 7402 including the light-emitting elements describedin Embodiment 1 or 2 and arranged in a matrix. The light-emittingelements can have high emission efficiency. Further, the light-emittingelements can be driven at low voltage. Furthermore, the light-emittingelements can have a long lifetime. Therefore, the mobile phone includingthe display portion 7402 which is formed using the light-emittingelements can have reduced power consumption. Further, the mobile phonecan be driven at low voltage. Furthermore, the mobile phone can havehigh reliability.

When the display portion 7402 of the mobile phone illustrated in FIG. 8Dis touched with a finger or the like, data can be input into the mobilephone. In this case, operations such as making a call and creating ane-mail can be performed by touching the display portion 7402 with afinger or the like.

There are mainly three screen modes of the display portion 7402. Thefirst mode is a display mode mainly for displaying an image. The secondmode is an input mode mainly for inputting information such ascharacters. The third mode is a display-and-input mode in which twomodes of the display mode and the input mode are combined.

For example, in the case of making a call or creating an e-mail, acharacter input mode is selected for the display portion 7402 so thatcharacters can be input on a screen. In this case, it is preferable todisplay a keyboard or number buttons on the screen of the displayportion 7402.

When a sensing device including a sensor such as a gyroscope or anacceleration sensor for detecting inclination is provided inside themobile phone, display on the screen can be automatically changed indirection by determining the orientation of the mobile phone (whetherthe mobile phone is placed horizontally or vertically).

The screen modes are switched by touch on the display portion 7402 oroperation with the operation buttons 7403. The screen modes can beswitched depending on the kind of images displayed on the displayportion 7402. For example, when a signal of an image displayed on thedisplay portion is a signal of moving image data, the screen mode isswitched to the display mode. When the signal is a signal of text data,the screen mode is switched to the input mode.

Moreover, in the input mode, when input by touching the display portion7402 is not performed for a certain period, the screen mode may becontrolled so as to be switched from the input mode to the display mode.Note that the touching operation may be sensed by an optical sensor inthe display portion 7402.

The display portion 7402 may function as an image sensor. For example,an image of a palm print, a fingerprint, or the like is taken by thedisplay portion 7402 while in touch with the palm or the finger, wherebypersonal authentication can be performed. Further, by providing abacklight or a sensing light source which emits a near-infrared light inthe display portion, an image of a finger vein, a palm vein, or the likecan be taken.

Note that the structure described in this embodiment can be combinedwith any of the structures described in Embodiments 1 to 4 asappropriate.

As described above, the application range of the light-emitting devicehaving the light-emitting element described in Embodiment 1 or 2 is wideso that this light-emitting device can be applied to electronic devicesin a variety of fields. By using the light-emitting element described inEmbodiment 1 or 2, an electronic device having reduced power consumptioncan be obtained.

FIG. 9 illustrates an example of a liquid crystal display device usingthe light-emitting element described in Embodiment 1 or 2 for abacklight. The liquid crystal display device illustrated in FIG. 9includes a housing 901, a liquid crystal layer 902, a backlight unit903, and a housing 904. The liquid crystal layer 902 is connected to adriver IC 905. To the backlight unit 903 is supplied current through aterminal 906.

The light-emitting element described in Embodiment 1 or 2 is used forthe backlight of the liquid crystal display device; thus, the backlightcan have reduced power consumption. In addition, the use of thelight-emitting element enables manufacture of a planar-emission lightingdevice and further a larger-area planar-emission lighting device;therefore, the backlight can be a larger-area backlight, and the liquidcrystal display device can also be a larger-area device. Furthermore,the light-emitting device can be thinner than a conventional one;accordingly, the display device can also be thinner.

FIG. 10 illustrates an example in which the light-emitting elementdescribed in Embodiment 1 or 2 is used for a table lamp which is alighting device. The table lamp illustrated in FIG. 10 includes ahousing 2001 and a light source 2002, and the light-emitting devicedescribed in Embodiment 4 is used for the light source 2002.

FIG. 11 illustrates an example in which the light-emitting elementdescribed in Embodiment 1 or 2 is used for an indoor lighting device3001 and a display device 3002. Since the light-emitting elementdescribed in Embodiment 1 or 2 has reduced power consumption, a lightingdevice that has reduced power consumption can be obtained. Further,since the light-emitting element described in Embodiment 1 or 2 can havea large area, the light-emitting element can be used for a large-arealighting device. Furthermore, since the light-emitting element describedin Embodiment 1 or 2 is thin, the light-emitting element can be used fora lighting device having a reduced thickness.

The light-emitting element described in Embodiment 1 or 2 can also beused for an automobile windshield or an automobile dashboard. FIG. 12illustrates one mode in which the light-emitting elements described inEmbodiment 1 or 2 are used for an automobile windshield and anautomobile dashboard. Displays 5000 to 5005 each include thelight-emitting element described in Embodiment 1 or 2.

The display 5000 and the display 5001 are provided in the automobilewindshield in which the light-emitting elements described in Embodiment1 or 2 are incorporated. The light-emitting element described inEmbodiment 1 or 2 can be formed into what is called a see-throughdisplay device, through which the opposite side can be seen, byincluding a first electrode and a second electrode formed of electrodeshaving light-transmitting properties. Such see-through display devicescan be provided even in the automobile windshield, without hindering thevision. Note that in the case where a transistor for driving or the likeis provided, a transistor having a light-transmitting property, such asan organic transistor using an organic semiconductor material or atransistor using an oxide semiconductor, is preferably used.

The display 5002 is provided in a pillar portion in which thelight-emitting elements described in Embodiment 1 or 2 are incorporated.The display 5002 can compensate for the view hindered by the pillarportion by showing an image taken by an imaging unit provided in the carbody. Similarly, the display 5003 provided in the dashboard cancompensate for the view hindered by the car body by showing an imagetaken by an imaging unit provided in the outside of the car body, whichleads to elimination of blind areas and enhancement of safety. Showingan image so as to compensate for the area which a driver cannot seemakes it possible for the driver to confirm safety easily andcomfortably.

The display 5004 and the display 5005 can provide a variety of kinds ofinformation such as navigation data, speed, axial rotation speed of anengine, a mileage, a fuel level, a gearshift state, and air-conditionsetting. The content or layout of the display can be changed freely by auser as appropriate. Note that such information can also be shown by thedisplays 5000 to 5003. The displays 5000 to 5005 can also be used aslighting devices.

The light-emitting element described in Embodiment 1 or 2 can have highemission efficiency and low power consumption. Therefore, load on abattery is small even when a number of large screens such as thedisplays 5000 to 5005 are provided, which provides comfortable use. Forthat reason, the light-emitting device and the lighting device each ofwhich includes the light-emitting element described in Embodiment 1 or 2can be suitably used as an in-vehicle light-emitting device and anin-vehicle lighting device.

FIGS. 13A and 13B illustrate an example of a foldable tablet terminal.FIG. 13A illustrates the tablet terminal which is unfolded. The tabletterminal includes a housing 9630, a display portion 9631 a, a displayportion 9631 b, a display mode switch 9034, a power switch 9035, apower-saving mode switch 9036, a clasp 9033, and an operation switch9038. Note that in the tablet terminal, one or both of the displayportion 9631 a and the display portion 9631 b is/are formed using alight-emitting device which includes the light-emitting elementdescribed in Embodiment 1 or 2.

Part of the display portion 9631 a can be a touchscreen region 9632 aand data can be input when a displayed operation key 9637 is touched.Although half of the display portion 9631 a has only a display functionand the other half has a touchscreen function, one embodiment of thepresent invention is not limited to the structure. The whole displayportion 9631 a may have a touchscreen function. For example, a keyboardcan be displayed on the entire region of the display portion 9631 a sothat the display portion 9631 a is used as a touchscreen, and thedisplay portion 9631 b can be used as a display screen.

Like the display portion 9631 a, part of the display portion 9631 b canbe a touchscreen region 9632 b. When a switching button 9639 forshowing/hiding a keyboard on the touchscreen is touched with a finger, astylus, or the like, the keyboard can be displayed on the displayportion 9631 b.

Touch input can be performed in the touchscreen region 9632 a and thetouchscreen region 9632 b at the same time.

The display mode switch 9034 can switch the display between portraitmode, landscape mode, and the like, and between monochrome display andcolor display, for example. The power-saving mode switch 9036 cancontrol display luminance in accordance with the amount of externallight in use of the tablet terminal sensed by an optical sensorincorporated in the tablet terminal. Another sensing device including asensor such as a gyroscope or an acceleration sensor for detectinginclination may be incorporated in the tablet terminal, in addition tothe optical sensor.

Although FIG. 13A illustrates an example in which the display portion9631 a and the display portion 9631 b have the same display area, oneembodiment of the present invention is not limited to the example. Thedisplay portion 9631 a and the display portion 9631 b may have differentdisplay areas and different display quality. For example, higherdefinition images may be displayed on one of the display portions 9631 aand 9631 b.

FIG. 13B illustrates the tablet terminal which is folded. The tabletterminal in this embodiment includes the housing 9630, a solar cell9633, a charge and discharge control circuit 9634, a battery 9635, and aDC-to-DC converter 9636.

Since the tablet terminal is foldable, the housing 9630 can be closedwhen the tablet terminal is not in use. As a result, the display portion9631 a and the display portion 9631 b can be protected, therebyproviding a tablet terminal with high endurance and high reliability forlong-term use.

The tablet terminal illustrated in FIGS. 13A and 13B can have otherfunctions such as a function of displaying various kinds of data (e.g.,a still image, a moving image, and a text image), a function ofdisplaying a calendar, a date, the time, or the like on the displayportion, a touch-input function of operating or editing the datadisplayed on the display portion by touch input, and a function ofcontrolling processing by various kinds of software (programs).

The solar cell 9633 provided on a surface of the tablet terminal cansupply power to the touchscreen, the display portion, a video signalprocessing portion, or the like. Note that the solar cell 9633 can beprovided on one or both surfaces of the housing 9630, so that thebattery 9635 can be charged efficiently.

The structure and operation of the charge and discharge control circuit9634 illustrated in FIG. 13B will be described with reference to a blockdiagram of FIG. 13C. FIG. 13C illustrates the solar cell 9633, thebattery 9635, the DC-to-DC converter 9636, a converter 9638, switchesSW1 to SW3, and a display portion 9631. The battery 9635, the DC-to-DCconverter 9636, the converter 9638, and the switches SW1 to SW3correspond to the charge and discharge control circuit 9634 illustratedin FIG. 13B.

First, description is made on an example of the operation in the casewhere power is generated by the solar cell 9633 with the use of externallight. The voltage of the power generated by the solar cell is raised orlowered by the DC-to-DC converter 9636 so as to be voltage for chargingthe battery 9635. Then, when power from the solar cell 9633 is used forthe operation of the display portion 9631, the switch SW1 is turned onand the voltage of the power is raised or lowered by the converter 9638so as to be voltage needed for the display portion 9631. When images arenot displayed on the display portion 9631, the switch SW1 is turned offand the switch SW2 is turned on so that the battery 9635 is charged.

Although the solar cell 9633 is described as an example of a powergeneration means, the power generation means is not particularlylimited, and the battery 9635 may be charged by another power generationmeans such as a piezoelectric element or a thermoelectric conversionelement (Peltier element). The battery 9635 may be charged by anon-contact power transmission module capable of performing charging bytransmitting and receiving power wirelessly (without contact), or any ofthe other charge means used in combination, and the power generationmeans is not necessarily provided.

One embodiment of the present invention is not limited to the tabletterminal having the shape illustrated in FIGS. 13A to 13C as long as thedisplay portion 9631 is included.

Example 1

In this example, a light-emitting element corresponding to oneembodiment of the present invention is described, in which platinum(II)octaethylporphyrin (abbreviation: PtOEP) is used as a substanceexhibiting thermally activated delayed fluorescence. Substances used inthis example are shown below.

Methods for fabricating a light-emitting element 1 and a comparativelight-emitting element 1 are described below.

(Method for Fabricating Light-Emitting Element 1)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 20 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2. Notethat the co-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Further, over the hole-transport layer 112, the light-emitting layer 113was formed by co-evaporation of2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (iii),3-[N-(9-phenylcarbazol-3-yl)-N-phenylamino]-9-phenylcarbazole(abbreviation: PCzPCA1) represented by Structural Formula (iv), andplatinum(II) octaethylporphyrin (abbreviation: PtOEP) represented byStructural Formula (v) with a weight ratio of 0.8:0.2:0.05(=2mDBTBPDBq-II:PCzPCA1:PtOEP) to a thickness of 40 nm.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 20 nm thick film of2mDBTBPDBq-II was formed and a 15 nm thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (vi) was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed. Lastly, aluminum wasdeposited by evaporation to a thickness of 200 nm to form the secondelectrode 102 functioning as a cathode. Thus, the light-emitting element1 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

(Method for Fabricating Comparative Light-Emitting Element 1)

The comparative light-emitting element 1 was fabricated in the samemanner as the light-emitting element 1 except the light-emitting layer113 was formed by co-evaporation of 2mDBTBPDBq-II and PtOEP with aweight ratio of 0.8:0.05 (=2mDBTBPDBq-II:PtOEP) to a thickness of 40 nm.The other materials and components are the same as those of thelight-emitting element 1.

The light-emitting element 1 and the comparative light-emitting element1 were each sealed using a glass substrate in a glove box containing anitrogen atmosphere so as not to be exposed to the air (specifically, asealing material was applied onto an outer edge of the element and heattreatment was performed at 80° C. for 1 hour at the time of sealing).Then, initial characteristics of these light-emitting elements weremeasured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 15 shows current density-luminance characteristics of thelight-emitting element 1 and the comparative light-emitting element 1;FIG. 16 shows voltage-luminance characteristics thereof; FIG. 17 showsluminance-current efficiency characteristics thereof; FIG. 18 showsluminance-power efficiency characteristics thereof; FIG. 19 showsluminance-external quantum efficiency characteristics thereof; and FIG.20 shows emission spectra thereof.

The figures show that the light-emitting element 1 which utilizes energytransfer from an exciplex exhibits better characteristics than thecomparative light-emitting element 1 which does not utilize the energytransfer. Specifically, as a result of an increase in external quantumefficiency and a decrease in voltage, the power efficiency and thecurrent efficiency are increased significantly. This confirms thesuperiority of the light-emitting element 1 that is one embodiment ofthe present invention.

Example 2

In this example, a light-emitting element corresponding to oneembodiment of the present invention is described, in which zinc(II)octaethylporphyrin (abbreviation: ZnOEP) is used as a substanceexhibiting thermally activated delayed fluorescence. Substances used inthis example are shown below.

Methods for fabricating a light-emitting element 2 and a comparativelight-emitting element 2 are described below.

(Method for Fabricating Light-Emitting Element 2)

First, a film of indium tin oxide containing silicon oxide (ITSO) wasformed over a glass substrate by a sputtering method, so that the firstelectrode 101 was formed. The thickness thereof was 110 nm and theelectrode area was 2 mm×2 mm. Here, the first electrode 101 is anelectrode that functions as an anode of the light-emitting element.

Next, as pretreatment for forming the light-emitting element over thesubstrate, UV ozone treatment was performed for 370 seconds afterwashing of a surface of the substrate with water and baking that wasperformed at 200° C. for one hour.

After that, the substrate was transferred into a vacuum evaporationapparatus where the pressure had been reduced to approximately 10⁻⁴ Pa,and was subjected to vacuum baking at 170° C. for 30 minutes in aheating chamber of the vacuum evaporation apparatus, and then thesubstrate was cooled down for about 30 minutes.

Then, the substrate over which the first electrode 101 was formed wasfixed to a substrate holder provided in the vacuum evaporation apparatusso that the surface on which the first electrode 101 was formed faceddownward. The pressure in the vacuum evaporation apparatus was reducedto about 10⁻⁴ Pa. After that, over the first electrode 101,4,4′,4″-(benzene-1,3,5-triyl)tri(dibenzothiophene) (abbreviation:DBT3P-II) represented by Structural Formula (i) and molybdenum(VI) oxidewere deposited by co-evaporation by an evaporation method usingresistance heating, so that the hole-injection layer 111 was formed. Thethickness of the hole-injection layer 111 was set to 20 nm, and theweight ratio of DBT3P-II to molybdenum oxide was adjusted to 4:2. Notethat the co-evaporation method refers to an evaporation method in whichevaporation is carried out from a plurality of evaporation sources atthe same time in one treatment chamber.

Next, a film of 4-phenyl-4′-(9-phenylfluoren-9-yl)triphenylamine(abbreviation: BPAFLP) which is represented by Structural Formula (ii)was formed to a thickness of 20 nm over the hole-injection layer 111 toform the hole-transport layer 112.

Further, over the hole-transport layer 112, the light-emitting layer 113was formed by co-evaporation of2-[3′-(dibenzothiophen-4-yl)biphenyl-3-yl]dibenzo[f,h]quinoxaline(abbreviation: 2mDBTBPDBq-II) represented by Structural Formula (iii),N,N′-bis(9-phenyl-9H-carbazol-3-yl)-N,N′-diphenyl-spiro-9,9′-bifluorene-2,7-diamine(abbreviation: PCA2SF) represented by Structural Formula (vii), andzinc(II) octaethylporphyrin (abbreviation: ZnOEP) represented byStructural Formula (viii) with a weight ratio of 0.8:0.2:0.01(=2mDBTBPDBq-II:PCA2SF:ZnOEP) to a thickness of 40 nm.

Then, the electron-transport layer 114 was formed over thelight-emitting layer 113 in such a way that a 20 nm thick film of2mDBTBPDBq-II was formed and a 10 nm thick film of bathophenanthroline(abbreviation: BPhen) represented by Structural Formula (vi) was formed.

After the formation of the electron-transport layer 114, lithiumfluoride (LiF) was deposited by evaporation to a thickness of 1 nm, sothat the electron-injection layer 115 was formed. Lastly, aluminum wasdeposited by evaporation to a thickness of 200 nm to form the secondelectrode 102 functioning as a cathode. Thus, the light-emitting element2 in this example was fabricated.

Note that in all the above evaporation steps, evaporation was performedby a resistance-heating method.

(Method for Fabricating Comparative Light-Emitting Element 2)

The comparative light-emitting element 2 was fabricated in the samemanner as the light-emitting element 2 except the light-emitting layer113 was formed by co-evaporation of 2mDBTBPDBq-II and ZnOEP with aweight ratio of 1:0.01 (=2mDBTBPDBq-II:ZnOEP) to a thickness of 40 nm.The other materials and components are the same as those of thelight-emitting element 2.

The light-emitting element 2 and the comparative light-emitting element2 were each sealed using a glass substrate in a glove box containing anitrogen atmosphere so as not to be exposed to the air (specifically, asealing material was applied onto an outer edge of the element and heattreatment was performed at 80° C. for 1 hour at the time of sealing).Then, initial characteristics of these light-emitting elements weremeasured. Note that the measurements were carried out at roomtemperature (in an atmosphere kept at 25° C.).

FIG. 21 shows current density-luminance characteristics of thelight-emitting element 2 and the comparative light-emitting element 2;FIG. 22 shows voltage-luminance characteristics thereof; FIG. 23 showsluminance-current efficiency characteristics thereof; FIG. 24 showsluminance-power efficiency characteristics thereof; FIG. 25 showsluminance-external quantum efficiency characteristics thereof; and FIG.26 shows emission spectra thereof.

The figures show that the light-emitting element 2 which utilizes energytransfer from an exciplex exhibits better characteristics than thecomparative light-emitting element 2 which does not utilize the energytransfer. Specifically, as a result of an increase in external quantumefficiency and a decrease in voltage, the power efficiency and thecurrent efficiency are increased significantly. This confirms thesuperiority of the light-emitting element 2 that is one embodiment ofthe present invention.

This application is based on Japanese Patent Application serial no.2012-114276 filed with Japan Patent Office on May 18, 2012, the entirecontents of which are hereby incorporated by reference.

What is claimed is:
 1. A light-emitting device comprising: a pair ofelectrodes; and a light-emitting layer between the pair of electrodes,the light-emitting layer comprising a first organic compound, a secondorganic compound, and a light-emitting substance which exhibitsthermally activated delayed fluorescence, wherein an emission of anexciplex of the first organic compound and the second organic compoundoverlaps with an absorption band of the light-emitting substance.
 2. Thelight-emitting device according to claim 1, wherein the emissionoverlaps with the absorption band on the lowest energy side.
 3. Thelight-emitting device according to claim 2, wherein a difference inenergy between a peak wavelength of the absorption band and a peakwavelength of the emission is 0.2 eV or less.
 4. The light-emittingdevice according to claim 1, wherein a difference in energy between peakwavelengths of fluorescence and phosphorescence of the light-emittingsubstance is 0.2 eV or less.
 5. The light-emitting device according toclaim 1, wherein the first organic compound has an electron-transportproperty, and the second organic compound has a hole-transport property.6. The light-emitting device according to claim 1, wherein the firstorganic compound is a π-deficient heteroaromatic compound, and thesecond organic compound is a π-excessive heteroaromatic compound or anaromatic amine compound.
 7. An electronic device comprising thelight-emitting device according to claim
 1. 8. A lighting devicecomprising the light-emitting device according to claim
 1. 9. Alight-emitting device comprising: a pair of electrodes; and alight-emitting layer between the pair of electrodes, the light-emittinglayer comprising a first organic compound, a second organic compound,and a light-emitting substance which exhibits thermally activateddelayed fluorescence, wherein a difference in energy between peakwavelengths of fluorescence and phosphorescence of the light-emittingsubstance is 0.2 eV or less, and wherein the light-emitting substancecomprises a π-excessive heteroaromatic ring and a π-deficientheteroaromatic ring.
 10. The light-emitting device according to claim 9,wherein a difference in equivalent energy between peak wavelengths offluorescence and phosphorescence of an exciplex of the first organiccompound and the second organic compound is 0.2 eV or less.
 11. Thelight-emitting device according to claim 9, wherein the first organiccompound has an electron-transport property, and the second organiccompound has a hole-transport property.
 12. The light-emitting deviceaccording to claim 9, wherein the first organic compound is aπ-deficient heteroaromatic compound, and the second organic compound isa π-excessive heteroaromatic compound or an aromatic amine compound. 13.An electronic device comprising the light-emitting device according toclaim
 9. 14. A lighting device comprising the light-emitting deviceaccording to claim 9.